The trucking industry is required to comply with regulations limiting the total weight and per axle load distribution imparted by a truck, including any cargo load, to a road surface. Typically a truck is loaded with cargo and then driven onto a weigh scale to measure the weight of each axle and the total combined weight.
Regulations specify the maximum total allowable weight permitted for a truck and also the maximum allowable weight per axle. Consequently, the trucker typically estimates the axle loads at time of loading the truck and drives the loaded to truck to a weigh scale facility prior to embarkation. The total weight of the truck load may be less then maximum total load limits while the distribution of the load among the axles may result in particular axles bearing excessive loads. Consequently efficient loading of the truck is a useful skill as adjustments to the distribution of the load are often time consuming and can be difficult once the truck is fully loaded.
The truck loading process has heretofore been guided primarily by experience and skill so as to distribute the load along a truck trailer or bed such that the total truck and axle weight regulations are satisfied while also maximizing the amount of cargo. Ideally, knowledge of the axle weights during the loading of the truck would facilitate the full loading of the truck so as to satisfy both objectives without the necessity of driving the truck to weigh scale facilities. Portable drive-on truck scales are sometimes carried with the trucks to facilitate axle load measurements at a loading site; however, this solution is usually not practical because of the inconvenience, size, weight and cost of the scales. Further, portable weigh scales when positioned on a surface present a vertical height offset such that when one set of wheels of a tandem axle are driven on to the scale, the load measurements must be corrected as the suspension springs of the measured wheel set are compressed more than the adjacent axle springs. Consequently, to obtain an accurate total weight of the vehicle one must utilize multiple portable axle scales located under each wheel set making portable scales impracticable for accurate on-site load measurement.
Alternatively, a dedicated apparatus mounted to each axle for load measurement eliminates the need to drive to a truck weigh scale facility. Incorporating a weigh scale apparatus permanently into a truck presents significant challenges. For a device to be mounted on the truck, the load measurement is accomplished by a transducer placed between the truck frame and the axle. For example, a transducer measuring load directly, such as a strain gauge, may be placed between the frame and the axle assembly. Alternatively, a linear displacement transducer may be positioned between the truck frame and the axle to measure the displacement of the frame relative to the undercarriage to measure the suspension compression. In either of these examples, electronic controllers are typically required to convert the transducer output signal into axle load information. The high capital cost of such controllers is often difficult to justify for dedicated installation. Additionally, for systems that are permanently mounted to the truck, the load transducer should be capable of tolerating both the large dynamic load changes and shocks presented by normal driving while also having sufficient sensitivity and capability to perform reliably and repeatability under the important static load conditions during truck cargo loading axle load measurement. Further, the harsh operating environment found between the truck frame and the road surface is particularly challenging for sensitive electronic transducers and the associated wiring and electronic controls. Road salts are aggressively corrosive to electronic components and wiring. Road debris requires transducers and sensors to be resistant to impact and particulate features.
Still other methods of axle scale load measurement include the incorporation of pneumatic bags in the suspension system wherein the air pressure in the bag increases with the compression of the bag in proportion to the axle loads. Alternatively, the pneumatic bag forms a portion of the shock absorbing or leveling. These measurement devices are typically integrated into the undercarriage suspension.
Accurate measurement of the axle loads, prior to the present invention, has required load measuring devices typically placed between the truck frame and the axle assemblies so as to measure the weight of a load and a truck frame. Load cells and other pressure measuring transmitters are typically mounted between the suspension and the axle. These systems are comprised of a transmitter element in electronic communication with a computing device and a data display element. The transmitters are exposed to high dynamic load changes during normal driving necessitating robust construction to achieve acceptable equipment reliability. Further, exposure to road debris, corrosives, oil and other hazards create a harsh operating environment especially for electronic components and wiring. Consequently economic and reliability constraints discourage the installation of axle load measuring devices.
What is needed is an improved truck axle scale being of low cost, simple installation, and incorporating robust engineering features capable of operation in environments present in and around the axle and carriage of a truck.